Island life prompts evolution of larger plant seeds

The seeds of the martinii variety of the shrub Coprosma propinqua are about one-fifth larger in area than those of the propinqua variety found on the New Zealand mainland. They follow a trend found for seeds of island-dwelling plants.

These changes happen because of the different conditions that occur on isolated pieces of land surrounded by vast stretches of water. Sometimes there’s an advantage to being smaller, such as when resources aren’t consistently available. For small species, moving to an island might lift some of the constraints that kept them tiny — their predators may be gone, for example.

Plants, too, can evolve along new paths when stuck on islands. And those paths lead to larger seeds, find Patrick H. Kavanagh and Kevin C. Burns of Victoria University of Wellington in New Zealand. Their study was published May 20 in the Proceedings of the Royal Society B.

Kavanagh and Burns looked at plants on four island groups off of New Zealand — the Three Kings Islands just to the north, the Kermadec Islands to the northeast, the Chatham Islands to the west and a couple of sub-Antarctic islands to the south. New Zealand itself is comprised of three islands, but these are the largest land masses in the area. They’ve been separate from other land for 80 million years. And all those little islands that surround them are populated by plants that started out in New Zealand.

The researchers collected the seeds of 40 island species from the four island groups and measured the seed sizes. They also gathered data from an atlas of New Zealand seeds.

The seeds from island species were larger than their New Zealand counterparts, the researchers found. The seeds of the martinii variety of the shrub Coprosma propinqua, for example, were about one-fifth larger than those of the propinqua variety found on the New Zealand mainland. And the tree Olearia chathamica had seeds about a third larger than its mainland match, Pleurophyllum criniferum. The differences weren’t a whole lot bigger — not, say, the different between an apple seed and a coconut seed — but they were consistent. And they were present for different types of plants and different types of seed dispersal.

A large seed may produce larger seedlings that have a competitive advantage. Those plants may even have a survival advantage in the long term. But bigger seeds don’t travel as far. And that can be a plus for an island plant, write Kavanagh and Burns. Long-distance travel isn’t a good thing if it means that a seed will just get lost at sea — that’s a wasted opportunity. Island plants need to keep their seeds close to home, and a bigger seed size helps to ensure that.